The present invention relates to a method for simultaneous recovery of pure benzene and pure toluene from hydrocarbon mixtures which contain these aromates.
More particularly, it relates to such a method in which the recovery is performed by extractive distillation with N-formylmorpholine and/or other N-substituted morpholines, whose substitutes contain not more than seven C-atoms as selective solvent. The entry product prior to introduction into the extractive distillation column is subjected to a predistillation, in which the ingredients which boil higher than the above mentioned aromates are separated as sump product.
The separation of aromates from hydrocarbon mixtures by extractive distillation with the above mentioned N-substituted morpholine as selective solvent is known for a long time. It is disclosed for example in the German patent 1,568,940. With the utilization of N-formylmorpholine as selective solvent, the above described operation have grown in numerous method production installations. When the entry product, in addition to benzene, also has high quantities of toluene as well as in some cases xylene, it is not advisable to subject this entry product directly to an extractive distillation. Due to the high boiling point of the toluene and the xylene, in this case the sump temperature in the extractive distillation column must be raised so that in the boiling region of these aromates also boiling non-aromates are completely evaporated from the sump of the extractive distillation. This increase of the sump temperature has the result that depending on the height of the temperature a high or average quantity of benzene escapes together with the non-aromates and the vapor through the head from the extractive distillation column. Thereby the benzene yield is correspondingly low.
In order to avoid this it is proposed in the German document DE-OS 1,543,119 to use a specific solvent without bond which, for the introduction of the entry product into the extractive distillation column, subjects the entry product to a predistillation in a manner that the components boiling higher than the aromates recovered are separated as sump products, while the precipitated aromate-containing head product is supplied without further separation into the extractive distillation column. In other words, in this method, depending upon the entry product and the method either benzene and toluene or benzene, toluene and xylene together with the corresponding boiling non-aromates are supplied into the extractive distillation columns.
This procedure which in practice is used with the above mentioned solvent, leads to the simultaneous recovery of benzene and toluene from a head product which contains these components. However, the extractive distillation column delivers a highly pure benzene. The non-aromate content of the recovered toluene lies however partially at approximately 1.5 weight percent. Since for predetermined applications a higher purity of the toluene is needed, these results are satisfactory only partially.